1. Field of the Invention
Embodiments of the invention relate generally to methods for forming tungsten films.
2. Description of the Related Art
Reliably producing sub-half micron and smaller features is one of the key technologies for the next generation of very large scale integrated (VLSI) circuits as well as ultra large scale integrated (ULSI) circuits. In particular, as the fringes of integrated circuit technology are pressed, the shrinking dimensions of interconnect features in VLSI and ULSI technology have placed additional demands on processing capabilities to increase both circuit density and the quality of integrated circuits. For example, multi-level interconnect features require careful processing of high aspect ratio structures (the ratio of the feature height to the feature width), such as vias, lines and contacts. It was thought that tungsten could not be extended to sub-half micron features or less. However, efforts are being made to extend the use of tungsten for interconnect metallization in such high aspect ratio structures.
As circuit densities increase, the widths of vias, lines and contacts may decrease to sub-quarter micron dimensions (e.g., less than about 0.2 μm), whereas the thickness of the dielectric material layers between such structures typically remains substantially constant, increasing the aspect ratios for such features. Many traditional deposition processes (e.g., chemical vapor deposition (CVD) and physical vapor deposition (PVD)) have difficulty filling sub-micron structures where the aspect ratio exceeds 6:1, and especially where the aspect ratio exceeds 10:1.
FIGS. 1A-1B illustrate the possible consequences of material layer deposition using conventional techniques in a high aspect ratio feature 6 formed on a substrate 1. The high aspect ratio feature 6 may be any opening such as a space formed between adjacent features 2, a contact, a via, or a trench defined in a layer 2. As shown in FIG. 1A, a material layer 11 that is deposited using conventional deposition techniques tends to be deposited on the top edges 6T of the feature 6 at a higher rate than at the bottom 6B or sides 6S thereof, creating an overhang. This overhang or excess deposition of material is sometimes referred to as crowning. Such excess material continues to build up on the top edges 6T of the feature 6, until the opening is closed off by the deposited material 11 forming a void 4 therein. Additionally, as shown in FIG. 1B, a seam 8 may be formed when a material layer 11 deposited on both sides 6S of the opening merge. The presence of either voids or seams may result in unreliable integrated circuit performance.
Therefore, a need exists for a method of depositing tungsten on a substrate to provide void-free and seam-free filling of high aspect ratio openings.